This invention relates generally to dynamoelectric machines and more particularly to end winding support assemblies of large turbine generators.
End winding assemblies have been variously constructed and been primarily designed to limit movement of the end turns relative to each other with the support assembly mounted in relatively fixed relation to the stator core. Part of the assembly has normally included a brace, such as of a glass fiber reinforced epoxy resin composition, in the form of a plate that extends in a radial plane on the outside of each end turn. The brace bears against the end turn and is held in place by a bracket that is rigidly secured to the brace as well as to the stator core. The bracket may be of cast metal.
The present invention addresses the specific problem of end winding assemblies, with brace and bracket elements, that are susceptible to vibration or oscillation at a resonance of the machine (such as 60 or 120 Hz) with a solution of that problem by modifying the previously rigid characteristic of the bracket in a manner that can be performed readily either in machines already in the field as well as in new manufacture.
A large machine such as a turbine generator can exhibit an unacceptably high vibration level in its end winding assembly if the assembly lacks axial flexibility and the machine is run at a natural frequency of the end winding assembly. If it is allowed to operate under such conditions, there is a susceptibility for the strands of the phase leads to become broken.
In some cases, the vibration level of an existing rigid end winding can be improved by removing some, such as each other one, of the coil support brackets that are normally provided radially outside each coil end turn. This somewhat reduces the total stiffness of the end winding but is generally not desirable because it leaves the end winding susceptible to damage from short circuit forces by introducing too much radial flexibility.
In accordance with the present invention, each coil end winding is held in place by a support bracket that is affixed to the core end face and also to a brace that is radially outside the end turn. As opposed to prior practice with totally rigid brackets, each bracket of the new arrangement has one or more resilient elements for allowing relative movement between a rigid portion of the bracket secured to the core and another rigid portion of the bracket secured to the brace. More specifically, the bracket has three elements that are arranged with two outside elements secured to the core and an intermediate element secured to the brace with mating surfaces of the adjacent elements having sheets of resilient material therebetween. The three elements of the bracket are secured together but allow a degree of flexure due to the resilient material being subjected to shear and compression forces so that there is a desired axial spring characteristic to change the natural frequency of the assembly thus avoiding excessive vibration at the running speed and frequency of the machine. The resilient bracket also allows for thermal expansion and contraction. A high degree of radial stiffness of the structure is retained, however.
The invention provides a way to achieve a controlled change of the stiffness in order to tune the end winding to a low vibration amplitude at running speed and frequency. This is achieved by the choice of the type of resilient material and its dimensions so that a wide range of stiffnesses are available. Also, the resilient support bracket, compared to previous rigid support brackets, increases the ability for the complete end winding to withstand thermal expansion relative to the core without affecting its ability to withstand short circuit loads. A further advantage is that the resilient support bracket can be easily installed on already built machines so as to minimize down time of a generator in the field by expeditiously retrofitting machines in the field that are found to have excessive vibration.